its similar to 'you are not the right fit' or 'we dont think this role is suitable for you'.

because somebody else is making that claim and not showing you the process and methods they used to reach to that conclusion.

if i said 'i do not believe you are cut out for this role, perhaps you ought to consider a more junior role instead' id need to back it up, what was my process and methods i used to reach to that conclusion ? am i just saying it ?

so thats why people can get offended when they're told 'you are not my type'.

Because it infers that you are deficient in some quality and that deficiency makes you unacceptable to me. This is quite offensive to a sensitive, insecure person. It may also sting a person who is confident in themselves.